A soliton wave.
A soliton wave is a confined energy phenomenon that travels at faster-than-light speeds, with potential applications in spacecraft propulsion.
In the 24th century, Doctor Ja'Dar of Bilana III investigated soliton waves as a possible alternative to warp drive. The USS Enterprise-D attended the first field test of this technology in 2368. The test involved the generation of a soliton wave using 23 field coils on the surface of Bilana III, which would envelop a small, unmanned vehicle and push it into warp towards Lemma II. There, a sister facility would generate a scattering field to dissipate the wave and bring the test ship out of warp.
Although the test began promisingly, showing a dramatic improvement in energy transfer efficiency of the soliton wave over the Enterprise's own warp engines, the wave unexpectedly destabilized and manifested a subspace distortion that destroyed the test ship and damaged the Enterprise. Dr. Ja'Dar believed that a transient power imbalance was responsible.
The Enterprise subsequently found that the wave was accelerating towards Lemma II, gaining energy in the process. It was estimated that by the time the wave arrived it would contain enough energy to destroy most of the planet. The wave had grown too powerful for the Lemma II facility to dissipate, but the Enterprise was able to disrupt the wave by detonating five photon torpedoes directly in front of it. (TNG: "New Ground")